What is Herbicide Resistance?

Herbicide resistance is the genetic capacity of a weed population
to survive a herbicide treatment that, under normal use conditions,
would effectively control that weed population. Herbicide resistance
is an example of evolution happening at an accelerated pace and
an illustration of the "survival of the fittest" principle.
A herbicide may kill all the weeds in a population of a particular
weed species except for a few individuals with the genetic capacity
to survive the herbicide. Herbicide resistant weeds are normally
very rare in a weed population. Applying the same herbicide in the
same field year after year will select for resistant plants. The
resistant weeds set seed and may eventually dominate the population.
This population is then not effectively controlled by the selecting
herbicide.

Resistant weeds may be resistant to only 1 herbicide group or to
2 or more herbicide groups. Also, resistant weeds may be resistant
to 1 herbicide class within a herbicide group or all of the herbicide
classes within 1 herbicide group. This is best illustrated with
the example below.

If a population of a certain weed species is resistant to Group
2 herbicides (ALS inhibitors) it may be resistant to one, several
or all of the herbicides that inhibit ALS (see Table
3). This is known as cross-resistance. Multiple resistance is
when the weed population is resistant to not only Group
2 herbicides but is also resistant to Group
5 (triazines) herbicides. To control this population of weeds
it would be necessary to select herbicides that are not included
in Group 2 or in Group 5
(see Table 3).

Example:

A pigweed population resistant to atrazine may also be resistant
to metribuzin (Sencor/Lexone) and simazine (Princep/Simadex) (cross-resistance).
If the same pigweed population is resistant to imazethapyr (Pursuit
- Group 2) it has multiple-resistance. [This
weed population may also be resistant to thifensulfuron-methyl (Pinnacle),
nicosulfuron or rimsulfuron (Accent, Ultim, Elim) or other Group
2 herbicides (cross-resistance).] However, herbicides from groups
other than Group 5 and Group
2, such as dicamba (Banvel, in Group 4)
or bromoxynil (Pardner, in Group 6), will
be as effective at controlling this population of weeds as they
are at controlling other susceptible populations of the same weed
species. Some herbicide products combine more than one active ingredient
(from different groups) as a strategy to delay resistance and to
control resistant weeds (for example: Broadstrike Dual Magnum, Fieldstar,
PeakPlus, Summit).

In Ontario, pigweed that are resistant to both triazine (Group
5 - examples: atrazine, metribuzin) and to ALS herbicides (Group
2 - examples: Pursuit, Classic, Pinnacle) have been identified.
Some populations of ryegrass in Australia are resistant to up to
10 different chemical groups. Multiple resistance significantly
reduces the options farmers have for the control of these weeds.

Multiple-resistance can appear after sequential selection. Green
foxtail from Manitoba is an example of sequential selection. Initially
this weed developed resistance to Treflan. Treflan resistance prompted
farmers to use postemergence grass herbicides (Group
1) as an alternative. After 4-5 years of post-emergence grass
herbicide applications, some foxtail populations became resistant
to these types of herbicides. The result was green foxtail resistant
to both Group 3 and Group
1 herbicides.

How Does Resistance Develop?

Worldwide there were more than 249 herbicide resistant weedy biotypes
in 47 countries. These numbers grow annually with new reports of
new resistant weeds. Some management practices increase the likelihood
of developing herbicide resistant weeds.

Resistance is more likely to occur when the same herbicide,
or herbicides from the same groups, are used repeatedly.

Monoculture often encourages the use of the same herbicide.

Resistance is most likely to develop in annual weed species
since they produce high numbers of seeds (pigweed, lamb's-quarters
and foxtail are good examples of these types of weeds).

Resistance frequently occurs to herbicides with the greatest
efficacy on a specific weed species. This is because they impose
intense selection on the weed species they are very effective
in controlling. The result is only the resistant individuals are
allowed to pass their genes to the next generation.

Herbicide mixtures and herbicide rotation strategies work on the
premise that if a weed carries the genes to resist 1 group of herbicides,
an alternate herbicide group will kill it. The difference between
the 2 approaches is that herbicide mixtures kill the resistant weed
using many active ingredients in the same season. Rotating herbicides
controls the resistant weeds in the years when effective herbicide
groups are used with the goal of reducing the resistant weed population.

Not only must mixtures contain herbicides from different groups,
but also each herbicide in the mixture must be acting on the same
weed species to effectively provide multiple modes of action. This
means that a mixture of a grass and broadleaf herbicide may make
sense in terms of overall weed control, but it may not be effective
as a resistance management tool. Some herbicides are a premix of
several herbicide groups, in other cases it is up to the farmer
and their advisors to select the best tank mix of products to provide
an affective weed resistance management strategy.

Note:

For these tactics to be effective it is important to select herbicides
with different plant-killing modes of action. Table
3 lists common herbicides used in Ontario grouped by their mode
of action and gives each group a unique number. This number system
is the same across North America. This list is used to help you
select products from different groups for use in rotations or mixtures.

A Self Test

Pick any one of your fields and fill out the crop rotation and
herbicide use history report below, Table 1.

Determine which herbicide groups were used in each year using
Table 3.

Compare the group numbers from 1 year to the next. More frequent
use of the same herbicide group may put your fields at higher
risk of developing herbicide resistant weed populations. Some
herbicides control weeds by attacking a single site of action,
such as Group 2, and may develop resistance
after being used 4-7 times. Others such as Group
5 do not seem to develop resistance until 10-15 years of repeated
use.

Include the next 2 years of your proposed crop rotation. This
allows you to evaluate future herbicide programs for resistance
management, as well as for soil residues and cropping restrictions.

Table 1.
Crop History for Field

Year

Crop

Herbicide

Group #

example 1

soybeans

Pursuit plus Basagran

2 and 6

example 2

corn

Marksman

4 and 5

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Weeds That are Currently Resistant

The following are examples of herbicide resistant weeds that
are known to exist in Ontario. In each case, growers have learned
how to adapt their weed management programs to include the resistant
weed biotypes.

First confirmed in 1974 and now found in
most counties of southern Ontario. Field histories in where
triazine resistant weeds have been found generally indicated
at least 6-10 consecutive years of triazine herbicide use.

These were confirmed in the late 1970's
and are now found in numerous locations across the province.
Resistant pigweed occurrences are not as prevalent as lamb's-quarters
but in areas where they occur they can have similar populations.
Resistant common ragweed is more localized but known to
occur in several areas of the province.

First confirmed in 1997, resistant pigweed
are now found in at least 8 counties.

Field histories indicated 3-5 years using Group 2
herbicides, not necessarily in consecutive years.

Not all populations are equally cross-resistant to
all Group 2 herbicides; however, it is not possible
to predict to which Group 2 herbicides a population
is resistant to without trial and error. It is prudent
to expect less than satisfactory control from most Group
2 herbicides and have alternative treatments in your
herbicide program.

At least one population of Group 2 resistant pigweed
is multiple-resistant to Group 5 (atrazine) herbicides.

Eastern black nightshade was confirmed in
2000 and found to be resistant to imazethapyr and possibly
to other group 2 herbicides

Herbicide groupings for Ontario follow the Weed Science Society
of America's nationally accepted grouping. Since groups 12, 13,
16, 17, 18, 21, 24, 25 and 26 are not available in Ontario, they
have been removed to simplify the chart. Taken from Table 3 in
OMAFRA Publication 75, Guide to Weed Control

What to do if You Find A Resistant Weed
Population?

Before declaring the weed resistant make sure that other explanations
for weed escapes and misses are investigated. Weeds that emerge
after application with non-residual herbicides can confuse the
diagnosis. Some species are naturally more tolerant to some herbicides.
Improper equipment setup, poor spray pattern, canopy penetration,
improper weed stage or weather issues can all lead to misses that
can be misdiagnosed as weed resistance.

When you do find resistance, the immediate answer is to use an
alternative herbicide for which there is no resistance. This may
mean adding another different herbicide to the spray tank, making
a second pass with an alternative product, or switching to a completely
different herbicide program. The main concern with these responses
to controlling a resistant weed population is that they may lead
to multiple resistance - as happened in Ontario with pigweed and
Manitoba with green foxtail.

Resistance is a consequence of relying too much on herbicides
for weed control. Changing herbicides may not be viable in the
long term if there is no change in the way weeds are managed globally.
Resistance management depends on many factors such as the herbicide
choice, the types of crops and the type of weed infestations.

When you have resistance, what do you do about it? If you don't
have resistance, how do you prevent it? For the most part the answers
to both questions are the same. Here are some ideas on managing
weed resistance.

Avoid using the same herbicide or herbicides from the same grouping
in the same field, in consecutive years.

Do not use the same mode of action more than
once per season. (Rescue sprays should be from a different herbicide
group.

Use tank mixes where 2 or more products give
effective control against the target weed, and the products
are from different mode of action groupings.

Use crop rotation, changing herbicides is not
enough. Different crops allow for a broader spectrum of herbicide
and tillage options to control weeds. Some crops are more aggressive
than others in competing against weeds.

Where possible use cultivation, cover crops,
or other practices to reduce viable weed seeds in the soil.

Integrated Weed Management is Resistance Management

Resistance delaying tactics such as mixtures and rotations are
most useful if they are made part of an Integrated Weed Management
(IWM) system. An IWM approach combines all available weed control
tools in the best possible way to manage weed populations while
maintaining economic crop production. In IWM, cultural and mechanical
weed control methods are complemented with chemical weed control.

Mechanical weed control may include inter-row cultivation or other
forms of tillage. Cultural control may be an important part of reducing
the over reliance on herbicides. This includes using varieties or
hybrids that are more competitive, seeding in narrow rows or planting
cover crops. Rotation is a part of crop management that may help
considerably in preventing resistance. Adding wheat to a corn/soybean
rotation can increase the opportunity to challenge weeds. Having
a crop that is planted at a different time and that is subject to
totally different management than corn and soybeans may have the
effect of destabilizing the weed populations. Weeds have a harder
time to adapt when management practices are changing.

The Bottom Line

Herbicides are very important tools for weed management Herbicides
provide efficient and cost effective weed control and should be
seen as resources that need to be protected. If they are overused,
resistance will make them obsolete. This is especially important
with the Group 2 herbicides because of their many desirable features
such as low use rates, low toxicity and high weed control efficacy.
By adopting an IWM approach, herbicides along with other methods
of control contribute to weed management. This may reduce the selection
pressure herbicides apply to resistant weeds.

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